The present invention relates generally to gas turbine engine combustors, and more particularly in one form of the present invention to a lean premix module which significantly reduces emissions of oxides of nitrogen while maintaining low emission levels of unburned hydrocarbons and carbon monoxide.
Air polluting emissions are an undesirable by-product from the operation of a gas turbine engine that burns fossil fuels. The primary air polluting emissions produced by the burning of fossil fuels include carbon dioxide, water vapor, oxides of nitrogen, carbon monoxide, unburned hydrocarbons, oxides of sulfur and particulates. Of the above emissions, carbon dioxide and water vapor are generally not considered objectionable. However, air pollution has become a worldwide concern and many countries have enacted stricter laws restricting the discharge of the pollutants from a gas turbine engine.
Gas turbine engine designers generally accept that many of the by-products of the combustion of a fossil fuel can be controlled by design modifications, cleanup of exhaust gases and/or regulating the quality of fuel. For example, the emission of particulates in exhaust gas have been controlled by design modifications to the combustors and fuel injectors, or by removing the particulates with traps and filters. The selection of fuels that are low in total sulfur content is a generally accepted method to control the discharge of sulfur oxides. Therefore the remaining polluting emissions of primary concern in the exhaust gases are oxides of nitrogen and unburned hydrocarbons.
The principal mechanism for the formation of oxides of nitrogen involves the direct oxidation of nitrogen and oxygen, and the chemical reaction producing this by-product occurs at a rate that is an exponential function of temperature. It is well known that in a gas turbine engine the oxidation of nitrogen is dependent upon the temperature in the primary combustion zone. Consequently, a small reduction in temperature within the combustor can result in a relatively large reduction in the emission of oxides of nitrogen. Further, in the traditional combustor, regions exist in the primary combustion zone that have stoichiometric mixtures with attendant high gas temperatures that enhance stability and combustion efficiency at the expense of oxides of nitrogen, carbon monoxide and unburned hydrocarbons production.
Until recently, a common technique for reducing the emission of oxides of nitrogen from a gas turbine engine involved reducing the flame temperature in the primary combustion zone of the combustor, such as through diluent injection which involves injecting large amounts of water or steam directly into the primary combustion zone. Diluent injection reduces the high temperatures that are produced in the stoichiometric regions of the current diffusion flame type combustors and the reduced temperatures reduce the formation of oxides of nitrogen. Unfortunately the lower temperatures slow the oxidation processes that are responsible for destroying unburned hydrocarbons and carbon monoxide thereby increasing their emission levels. Further, diluent injection also negatively impacts combustor and turbine durability and a fuel consumption penalty is incurred.
While diluent injection has been utilized to reduce the formation of oxides of nitrogen, a second technique selective catalytic reduction has been utilized to convert the oxides of nitrogen into nitrogen gas after it is formed. Both of these prior techniques have the disadvantages of added complexity, high installation costs, high operating costs and reduced engine reliability.
Most recently, gas turbine engine designers and manufacturers have generally adopted a lean premix combustion technique to reduce the pollutant emissions from the engine by altering the basic combustion process where the pollutants are formed, thereby making the combustion process inherently clean. In lean premix combustion the fuel and air are premixed to a fuel lean proportion prior to combustion. The premixing of the fuel and air in this fashion avoids the high temperature stoichiometric fuel air mixtures which yields the corresponding highest flame temperatures, and therefore the formation rate of oxides of nitrogen, which is exponentially dependent, on temperature is lowered.
Although the prior techniques for reducing the emissions of oxides of nitrogen from gas turbine engines are steps in the right direction, the need for additional improvements still remain. The present invention satisfies this need in a novel and unobvious way.